The present invention relates to capacitative ultrasound transducers that are manufactured micromechanically and, thus, comprise a low-mass membrane.
Capacitative ultrasound transducers have a low-mass membrane for sound emission. The pre-condition for great bandwidths in the sound emission having good efficiency is thus established. The emitted power is determined by the driving force and the membrane stroke resulting therefrom. Capacitative transducers are currently preferably utilized for the emission of ultrasound in air.
Given this sound emission, a corresponding stroke is present for a given drive force for the membrane. When the sound emission is then considered in fluids, then a significantly smaller membrane stroke is achieved given the same force than in emission at or in air. The emitted power of the ultrasound transducer and the efficiency thereof thus drop. This is true both for fluids as well as for biological tissue that exhibits similar properties in this application. For a meaningful utilization of the capacitative transducer principle in fluids and biological tissues, it is necessary to reduce the distance between the electrodes of the capacitor representing the ultrasound transducer xe2x80x94the capacitor gap. As a result thereof, a higherfield strength given the same electrical voltage and, thus, a higher drive force are achieved. In the ideal case, the increased drive force causes a membrane stroke that is nearly as great as the height of the capacitor gap. The gap should be maximally 20 nm high, particularly for high frequencies around 50 MHZ and a voltage of 10 V. Such slight heights, however, cannot be currently manufactured.
In the prior art, other transducer principles such as, for example, the piezo-electrical principle are mainly utilized for generating sound in fluids and biological tissues. Given a micromechanical manufacturing process, what is crucial is to manufacture of the extremely small air gap between membrane and cooperating electrode. Previous micromechanical solutions involve a gap height of approximately 100 nm.
The invention is based on an object of offering a micromechanically manufactured, capacitative ultrasound transducer that comprises a gap height of less than 100 nm. Manufacturing methods are also disclosed.
In an embodiment, the present invention provides a micromechanical, capacitative ultrasound transducer that comprises a cooperating electrode planarly applied on a substrate, the cooperating electrode having a central region. The transducer also comprises a spacer layer disposed on the substrate and/or on the cooperating electrode. The spacer has at least one recess disposed over the central region of the cooperating electrode thereby leaving it exposed. The transducer also includes a micromechanical membrane applied on the spacer layer and which covers the recess of the cooperating electrode. The micromechanical membrane has a side that faces towards the cooperating electrode. This side of the membrane comprises a plurality of nubs. The micromechanical membrane is held down by a photoresist layer or by an electrically applied force so that the nubs of the side of the micromechanical membrane engage the cooperating electrode.
In an embodiment, the electrically applied force is generated by applying a DC voltage.
In an embodiment, a spacing between the nubs and the cooperating electrode is formed due to resilient properties of the photoresist layer and the spacing can be eliminated by an electrically applied force.
In an embodiment, the nubs encompass a recess.
In an embodiment, the nubs are circularly arranged.
In an embodiment, the nubs have a uniform height ranging from 10 nm to 100 nm.
In an embodiment, a central region of the membrane comprises at least one stiffening layer disposed on a side of the membrane lying opposite the nubs.
In an embodiment, the cooperating electrode comprises polysilicon.
In an embodiment, a plurality of micromechanical ultrasound transducers are positioned neighboring one another to form a two-dimensional transducer arrangement.
The present invention also provides a method for manufacturing the micromechanical ultrasound transducer described above. As a hold down-means for the membrane, a photoresist layer is applied and thereafter photolithographically removed in the central region of the cooperating electrode. The ultrasound transducer is charged with gas pressure in an autoclave so that the nubs are pressed against the cooperating electrode and the photoresist layer is simultaneously cured.
In an embodiment, a gap between the nubs and the cooperating electrode arises when the pressure is removed, and this gap is closed by the application of a DC voltage.
The invention is based on the perception that capacitative ultrasound transducers having an air gap of, for example, 600 nm that are manufactured in a micromechanical standard process can be modified such that the desired properties can be realized. An ultrasound transducer manufactured with a planar membrane is thus subjected to a procedure that assures a desired, extremely small gap height between the capacitor electrodes.
To this end, nubs that exhibit a uniform height are applied on the membrane side that lies opposite the cooperating electrode. Their height lies in the range from 10 through 100 nm.
The membrane is deformed such from the planar structure in this method that the nubs thereof lie against the cooperating electrode. This is true for the central part of the membrane or, respectively, of the cooperating electrode located in or over a recess. The nubs thereby lie firmly against the cooperating electrode. The region between the nubs and/or the region between the nubs and a clamping of the membrane is respectively available as elastic membrane region for sound emission. It is thereby to be taken into consideration that the use of a photoresist has hold-down means essentially covers the lateral regions between the nubs and the membrane clamping and renders this ineffective.
Overall, a micromechanically manufactured ultrasound transducer having a planar membrane is developed such that its membrane is partially collapsed. In this condition, the membrane is attracted or, respectively, arrested such that it lies against the cooperating electrode. This condition can ensue electrically or mechanically. Given the disclosed ultrasound transducer, the physical contact between membrane and cooperating electrode is represented by the nubs located on the membrane. A complete collapse wherein zones of the membrane located next to the nubs are also in contact with the cooperating electrode is avoided.
Since the cooperating electrode is composed of a material, usually silicon, that superficially oxidizes, no short arises. In this collapsed condition, a uniform air gap having minimum height is present. Given a corresponding electrical excitation with, for example, 50 MHZ and 10 V, one can count on a correspondingly increased drive force for the membrane.
A gap between nubs and cooperating electrode subsequently recurring during manufacture due to resiliency of the photoresist can be preserved dependent on the application or can be advantageously closed by an electrically applied force.
Other objects and advantages of the present invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.